The growth in petroleum and natural gas processing around the world has led to an increased awareness about the amount of gas being flared into the atmosphere. Not only does flaring pose an environmental concern due to the amount of carbon dioxide being released (some 400 million tons worth), but it also represents a major waste of natural resources and money. In North Dakota, for example, it is estimated that about $1 billion worth of C3+ hydrocarbons, also known as natural gas liquids, are flared per year rather than being recovered on-site. Globally, the number is estimated to be close to $20 billion.
The lack of investment in infrastructure needed to handle large amounts of natural gas or associated gas from oil reservoirs contributes to the issue of flare waste. The building of gas pipelines is usually given lesser priority over the development of more lucrative oil wells. Additionally, these oil wells are typically in remote locations where there is no local market for sales gas. While legislative measures are being taken by governments around the world to promote the use of flare-reduction technology, the systems currently in place are large, costly and energy-intensive.
A conventional way to recover C3+ hydrocarbons from flare gas is shown in FIG. 1.
Referring to this figure, a relatively low-pressure flare gas, 101, comprising mostly of methane and C3+ hydrocarbons, but also including ethane, nitrogen, acid gases, and water vapor, is first sent to a dehydration unit, 102, to remove water-enriched stream 120. Any water or hydrates present in the flare gas must be removed prior to refrigeration in order to avoid fouling downstream processes that operate at below freezing temperatures. A dehydrated gas stream, 103, is withdrawn from dehydration unit, 102, and is passed to a compressor, 104, which produces a compressed stream, 105.
Next, compressed stream 105 is directed to a first separator, 106. The first separation unit is typically a gas-liquid phase separator or the like that separates the incoming compressed stream into a first hydrocarbon liquid stream, 108, containing C3+ hydrocarbon and a first gas stream, 107, depleted in C3+ hydrocarbons.
The first gas stream, 107, is then sent to a refrigeration process, 109, in order to condense additional amounts of C3+ hydrocarbons. The refrigeration process, 109, is typically operated by further compression in a second compression step, then chilled down to about −20° C. or −40° C., depending on pressure, or by cooling under cryogenic conditions to about −100° C. Typically, external refrigerants are used to cool the natural gas stream, and then, an expansion turbine is used to rapidly expand the chilled gases, thereby causing significant further cooling. This rapid temperature drop condenses C3+ hydrocarbons in the gas stream, while maintaining methane in gaseous form. However, turboexpanders require a large capital investment and, depending on the process, may not be an economically viable option.
Alternatively, cooling may be achieved by using Joule-Thomson expansion across a pressure-reducing valve. However, this method is not effective to achieve significant cooling unless the incoming gas is at very high pressure, so that a very large pressure differential across the valve can be obtained.
After undergoing refrigeration, the cooled gas stream, 110, is sent to a second separation unit, 111. First hydrocarbon liquid stream 108 is also sent to this unit. The second separation unit, 111, separates incoming streams 108 and 110 into a second hydrocarbon liquid stream, 112, which is enriched in C3+ hydrocarbons, and a second gas stream, 115, that is depleted in C3+ hydrocarbons.
The second hydrocarbon liquid stream, 112, is passed to a blower (or compressor), 113, before entering a distillation column, 116. The distillation column, 116, is typically a deethanizer or demethanizer, from which a third hydrocarbon liquid stream, 119, enriched in C3+ hydrocarbons, is removed as a bottoms product.
A third gas stream, 117, which is depleted in C3+ hydrocarbons, is removed from the top of distillation column 116 and is eventually mixed with second gas stream 115 to produce mixed gaseous stream 118, which may be flared or used as fuel or sales gas.
A variety of NGL recovery processes are known in the literature and are described in detail, below.
U.S. Pat. No. 5,352,272 to Moll et al. describes processes for operating certain glassy membranes selective for carbon dioxide over methane at temperatures of 5° C. or below. The patent shows an example, Example 10, in which self-refrigeration of the feed gas to the desired operating temperature is provided by Joule-Thomson cooling.
Co-owned U.S. Pat. No. 5,501,722 to Toy et al. discloses a membrane having a selective layer comprising poly(trimethylsilylpropyne) that is selectively permeable to C3+ hydrocarbons over methane. The membrane may be used in NGL recovery from refinery gases or off gases from the petrochemical industry.
U.S. Pat. No. 5,685,170 to Sorenson et al. discloses an NGL recovery process using an absorber employed upstream of an expansion device, such as a turboexpander or a J-T valve, wherein the cooled vapor streams from the absorber are combined with the cooled and expanded vapor stream of a downstream distillation column. While such configurations advantageously make use of the pressure in the feed gas, a gas dehydration unit must be installed for the cryogenic expander operation, and residue gas in such plants needs to be recompressed which negates any cost or energy savings.
Co-owned U.S. Pat. No. 6,053,965 to Lokhandwala et al. describes a fuel gas conditioning process that may be used to produce an NGL product. According to the general process disclosed in the '965 patent, a portion of gas from a high-pressure gas stream is withdrawn, then cooled by passing the portion through a heat-exchange step in heat-exchanging relationship against a membrane residue stream. The portion is then separated into a liquid phase comprising C3+ hydrocarbons (NGL) and a gas phase depleted in C3+ hydrocarbons. The gas phase is then passed across the feed side of a membrane unit containing a membrane selective for C3+ hydrocarbons over methane. A C3+-depleted membrane residue stream is then withdrawn from the feed side and passed back to the heat exchange step. A C3+-enriched permeate stream is withdrawn from the permeate side. The membrane residue stream may optionally be used as combustion fuel for a prime mover.
U.S. PGPUB 2012/0096895 to Patel et al. teaches a process using a combination of expansion, separation, and compression sequences to recover NGL without the need for a demethanizer column. To provide expansion cooling, the process uses J-T valves or turboexpanders.
While improvements have been made in NGL recovery processes, the current technology still relies on the use of expensive refrigeration/cryogenic equipment. Given the need to develop technology for reducing flare gas waste and the current costs of prior art processes, it would be desirable to have a cheaper and more efficient process for enhanced recovery of NGL.